When it is desired to conference a number of telephone lines it is customary to use 4-wire operation where the transmit and receive communication signals are on individual wire pairs. The wire pairs from each line are then selectively interconnected through a common amplifier by a switching matrix with the crosspoints of such matrix having the capability of switching all four wires of each line. Such a switching matrix when constructed electromechanically is inherently bulky and when electronics are used to reduce the size the need for four crosspoints per line circuitry becomes prohibitively expensive.
Two-wire conferencing is possible with series and parallel negative impedance, however, the negative impedance must be changed for different conference sizes and configurations. Control of the negative impedance can be awkward.
Another problem exists when three or more telephones are connected together in conference fashion with no special conferencing circuitry. In such a situation a loss of signal strength is experienced compared with the signal strength in a typical two telephone connection. As each additional telephone station is added to the conference the signal strength is reduced proportionately. When the crosspoints of the switching matrix have zero resistance, the loss in signal strength is caused by the additional impedance load of each added telephone station.
One arrangement for solving these problems is disclosed in U.S. patent application Ser. No. 580,292 filed May 23, 1975 by D. J. Morgan and D. C. Smith, now U.S. Pat. No. 3,991,279 . In the Morgan et al disclosure there is shown a monobus port circuit interposed between a transmission line and a switching matrix.
The interface circuit is a current source arranged to provide the current equivalent of the voltage signal available from the transmission source. One bus resistor is supplied by the switching matrix and is shared in common by the monobus circuits serving the other connected transmission lines. In this manner the current signal generated by the port circuit current source is converted to a voltage signal by flowing through the common bus resistor. This bus voltage signal is supplied via the switching matrix to each connected port circuit.
When the switching matrix crosspoint resistance is negligible, the component of the bus voltage measured at the current source output which results from current generated from each port circuit is equal and opposite in magnitude to the voltage generated in that port circuit. In any given port circuit these voltages cancel each other, thereby eliminating the feedback signal.
Since for unbalanced operation the output from each port can be a single wire, the switching matrix is reduced to one device per crosspoint without inherently limiting the number of lines which can be interconnected concurrently.
One problem with such a monobus port circuit is that a transformer and hybrid is necessary to interface the telephone line and the voltage to current conversion circuitry. Thus, while the current conversion and feedback signal cancellation circuits are electronic in nature the actual telephone line transformer interface is bulky and relatively expensive.